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Day 3 of 5
⏱ ~60 minutes
Networking for Developers — Day 3

TCP/IP and Ports

IP addresses, subnets, ports, TCP vs UDP, and practical tools: curl, netcat, and nmap.

IP Addresses and Ports

IP Addresses
# IPv4: 4 octets, each 0-255
192.168.1.100
10.0.0.1         ← private (10.x.x.x)
172.16.0.1       ← private (172.16-31.x.x)
192.168.0.1      ← private (192.168.x.x)
127.0.0.1        ← loopback (this machine)

# IPv6: 128 bits
2001:0db8:85a3::8a2e:0370:7334

# Check your IP
curl ifconfig.me          # public IP
ip addr show              # local IPs (Linux)
ipconfig                  # local IPs (Windows)
Ports
# Well-known ports
22    SSH
25    SMTP
53    DNS
80    HTTP
443   HTTPS
3306  MySQL
5432  PostgreSQL
6379  Redis
27017 MongoDB

# Check open ports
ss -tlnp          # listening TCP ports
nmap localhost    # scan local ports

# Test if a port is open
nc -zv hostname 443
curl -v telnet://hostname:443
TCP vs UDP
# TCP: connection-oriented, reliable
# - Handshake before data
# - Guarantees delivery and order
# - Used for: HTTP, HTTPS, SSH, databases

# UDP: connectionless, fast
# - No handshake, fire and forget
# - Faster, lower overhead
# - Used for: DNS, video streaming, gaming, VoIP

# Test TCP connection
curl -w 'Connected in %{time_connect}s\n' -o /dev/null -s https://github.com
📝 Day 3 Exercise
Map Your Ports
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Day 3 Summary

  • IP addresses: private (10.x, 192.168.x, 172.16-31.x) vs public. 127.0.0.1 = this machine.
  • Ports identify services. 80=HTTP, 443=HTTPS, 22=SSH, 5432=Postgres.
  • TCP = reliable, ordered, with handshake. UDP = fast, no guarantee. HTTP uses TCP.
  • ss -tlnp shows listening ports. nc -zv host port tests connectivity.
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Day 4: DNS: How Names Become IPs
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